The primary purpose of amplifiers is to faithfully reproduce only those signals applied to the input. In practice, however, extraneous signals can enter the amplifier circuit through the DC power supply, are electromagnetically induced into the amplifier circuit or are generated by the amplifier components themselves. While the noise or signal problems caused by the latter two mentioned situations can be dealt with by proper selection, placement and shielding techniques, the noise and extraneous signals arising from the power supply circuitry are more difficult to eliminate or reduce.
In many applications, amplifier demands require a high power supply rejection ratio (PSRR), especially in environments having an abundance of noise and extraneous signals. This is especially true in telephone switching circuits where digital switching equipment is interfaced with analog voice equipment to either code analog voice signals into a digital transmission format, or perform the reverse decoding operation. Notwithstanding the high noise content prevalent in the digital and analog conversion process, operational amplifiers are required to present a high immunity to such extraneous power supply signals.
An amplifier's immunity to extraneous signals becomes even more important when operating at high frequencies. As the signal frequency increases the feedback of the amplifier is unable to completely correct itself, thus the gain decreases. Near the unity gain bandwidth point, the amplitude of extraneous signals may become significant when combined with high frequency signals of reduced amplification.
One approach commonly used to increase the signal immunity on the power supply inputs of operational amplifiers is to use R-C filters connected to the positive (V+) and negative (V-) operational amplifier power supply inputs. While this approach is relatively inexpensive and easy to implement with discrete components, the solution is generally impractical in monolithic circuits as the cost of the capacitive element would be prohibitive.
Another approach previously taken to increase the noise and signal immunity of an operational amplifier is to provide a power supply for the small signal stage thereof, and another power supply for the output or drive stage. The small signals or intermediate stages of an operational amplifier are generally the high gain stages, and thus are critical stages with regard to extraneous signal immunity. However, dual power supplies are expensive and, in many applications, are also impractical to implement.
The low immunity of operational amplifier stages to extraneous signals is due in a large part to circuit design. More specifically, the major contributors to poor extraneous signal immunity are forward biased PN junctions (in bipolar circuits) and source to gate structures in MOS transistor circuits. This is especially true when such junctions and structures appear as low impedance paths between the operational amplifier circuit and the power supply inputs. Any signal or noise component within the power supply is thus easily coupled into amplifier circuitry.
From the foregoing, it may be seen that a need has arisen for an amplifier design which addresses noise and signal rejection in amplifiers without the accompanying shortcomings attendant with prior techniques.